Screw thread



o C C mi@ Jsd E e Aug. 31, 1937.

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g BY' Q ATTORNEYS Petenied Aug. 31,` 1937 SCREW THREAD Edward J.McManus, Orange, N. J., assignor to Dardelet l'I'lireadlock Corporation,`New York, N. Y., a corporation of Delaware Apeueetien March zo. 193s,serial No. casas a einem.V (ci. isi- 14) This invention relates moreparticularly to im provements in self-locking screw threads of theDardelet type disclosed in U. S. Patent No. 1,657,244.

An important object of the invention is to provide a Dardelet screwthread with an improved of a complementary threadfor screw l,engagementwith the improved thread. l

Other objects of the invention will appear hereinafter.

In the drawing', Fig. 1 is an enlarged detail sectional view of anexternal screw thread embodyingv theimproved profile and with aninternal screw thread engaged therewith, said threads being in unlockedrelation;

Fig. 2 is a' view similar to Fig. 1 showing the threads in mutuallyself-locked relation:

Fig. 3 is a side elevation of an end portion of a bolt embodying 'theimproved thread; and

' Figs. tand 5 are views similar to Figs. 1 and 2 respectively showingDardelet threads of a standard, commercial form in unlocked and lockedrelation.

As shown in Figs. 4 and 5, a standard, commercial form ofDardeletexternal screw thread, such as that of a bolt, has a rootlocking surface lll4 which, in cross-section, extends the full width ofthe bottom of the thread groove. This surface slopes slightly withreference to the thread axis, the rise of its slope being in thedirectionof the tip of the bolt, that is, away from the bolt head. Aninternal thread, such as that--of a nut,

complementary to said external thread, has along itscrest a lockingsurface Il with a slope similar to that of the root surface I0. Thedegree of slope is within the angle of friction or the angle of reposeof the two locking surfaces. In standard Dardelet threads the slope issix degrees.

locking engagement with the sloping locking surl face i0 of the boltthread, as shown in Fig. 5. The sloping surface l0 is directly opposedto the surface Il from start to nish of the movellowahle variation insize, in the forming of the is to materially increase the range of saidtolerment of surface Il from unlocked to locked position. .Consequentlywhen surface Il is displaced to the right for locking, its entire areamakes a broadside engagement with the surface IIJ and imposes afsuddenresistance which lis liable to fool the-workman into thinking thatthe'connection is tightened. Much force is required to fully tighten theconnection because of the drag of said full, broadside engagement fromthe time surface Il engages surface I0 until the locking '10 limit isreached and abutment i3 engages abutment I2, as shown in Fig. 5. In thescrewing of the nut up to the abutment, 'and in the unscrewing of thenut, the thread of the nut is displaced in the opposite direction todispose its locking surface il out of contact with the *locking surfacel0 for free rotation of the nut. The rib of the bolt thread has sidefaces I2 abrupt with reference to the thread axis, and the rib of thenut thread has similarly abrupt side faces i3 to abut the faces i2 andpositively limit thread displacement in both directions. In the stand'-ard, commercial Dardelet thread the surfacesv I2 and i3 are eachdisposed at an angle of fourteen and one-half degrees to a line normalto the thread axis.

Threads of the typejust described require very accurate shaping forproper functioning. For free screwing and unscrewing it is necessarythat there be clearance between the locking surfaces Ip and il when thethreads are in the relative positions shown in Fig. 4. 'At the same timethe allowable clearance is limited bythe necessity for adequate lockingeiect when the threads are in the relative positions shown in Fig. 5.Consequently the range of tolerance, or maximum alsurfaces I0 and Il isquite limited. In fact the said tolerance is limited to a fewthousandths of an inch for quite a wide range of thread sizes. 40' Animportant .purpose of the present invention ance in a simple manner,with a minimum deviation from a Standard commercial Dardelet threadprole, without sacrice-of *thread-locking effect and in factv withincreased locking effect in some cases.

The improved co-action of the threads in frictional locking engagement,and the advantages of increased tolerance arev obtained by animprovement in the profile of the external thread, or the thread whichlhas the sloping locking surface at its root. An external thread i4embodying this improvement is shown in Figs. 1, 2 and 3. There, in orderto ensure the gradual sion exceeds the Width of. the rib crest II oflthe complementary thread by several thou-- sandths of an inch, and thedepression is deeper in the groove than the lowest point of the lockingsurface Ina. A side Wall I1 of the depression connects the bottom of thedepression with the adjacent low edge of the slopingF surface IOS. Saidwall I'I may be normal to the thread axis or it may slope, as shown. Itshould be much more abrupt to the thread axis than the surface I0 inorder to avoid contact with the adjacent corner of the thread crest IIwhen the threads are in their unlocked relation, as shown in Fig. 1.'I'he depth of the relief depression may be only several thousandths ofan inch below the surface IIIn inporder to afford adequate relief,without appreciable sacrifice of tensile strength in the bolt.

In Fig. 1 the crest II of the internal thread is shown in a positionoverlying the relief depression and coincident or substantiallycoincident with the prolongation of the` line of slope of the surfaceI0, and entirely t of contact with the external thread. 'Ihe sition ofthe crest I I may vary to thousandths of an inch without impairing thelocking cooperation of the threads and also without Ahindering freescrewing and unscrewing when the threads are in their unlocked relation.It is also 4possible to vary the position of the surface Ill tothousandths. of an inchv relatively to the crest surfacel I, withoutobjectionable effect upon the screwing and unscrewing of the threads.Due to the provision of the relief depression either of. the lockingsurfaces `III and II may be safely raised or lowered relatively to theother without objectionable effect. It has been found by trial that ifthe crest II is so pitioned as to strike the sidewall Il of thedepression in the locking shift of one thread on the other there occursno objectionable resistance to the riding of .the crest II onto thesurface IUS. The invention provides a materially greaterv range oftolerance in the shaping of the threads than is afforded by the threadsprofiled as in Figs. 4 and 5.

The depression or channel I6 and the associated sloping locking surfaceIIIIL are so formed and larranged with reference to each other and tothe crest rib 'locking surface II as to afford a radial tolerance whichpermits relative radial variation of the positionsrof both of 'saidlocking surfaces when unlocked, from one relation in which the crestsurface II is disposed intermediate the depth of the channel I6 toanother relation in which the crest surface II is spaced radiallyoutside of the channel and in axial alinement with a point intermediatethe rise of the sloping parts in the unlocking operation, and at thesame time also provide clearance for free sloping root locking surfaceIIJa and still secure' screwing when the threads are in unlockedrelation. 'Ihe channel I6 is a tolerance-augmenting element, and thesloping locking surface IIIn when associated with the channel becomesanother tolerance-augmenting element.

Referring to Fig. 1 it will be seenvthat when the threads are unlockedthe surface II is entirely displaced from the surface IDH. When thedisplacement of surface I I to the right for locking action occurs'onlya small marginal area of the surface II will at first engage the surfaceIII. 'Ihe remainder of surface II will still overhang the helicoidaldepressionv I6 and impose no drag resistance. strong, abruptly imposedresistance to the movement of surface II. As the displacement of thesurface lII to the right continues, more and more of the area of saidsurface is brought into engagement with surface I0". The screwingresistance, due to the increase in compression by wedging effect of theslope of the surfaces I I and Consequently there is nov I0* and due tothe drag of increased area of surf interchangeability of Dardeletthreads of the same pitch and diameter. It enables complementary threadsto be shaped for more positive mutual locking effect without sacrificeof free screwing and unscrewing when the threads are in unlockedrelation. It facilitates and cheapens the manufacture of both externaland internal threads. In the case of the cutting of the bolt thread iteliminates much adjustment of the chasers or.dies employed. In'thecutting of the nut thread afmaterially lgreater wear-down of the tap ispermissiblefwith consequent increased production obtainable from thetap. It renders more practicable the forming of Dardelet threaded boltsby hot rolling. It allows for electroplating with cadmium nickel andchromium, for example, or for galvanizing the threads withoutobstructing their working cooperation. It enables threads to be heattreated without danger of distortion beyond a satisfactory working fit.It, in general, extends the range of materials and the range ofprocesses which may be employed in the manufacture of the threadswithout danger of faulty fitting of complementary external and internalthreads.

Another important advantage which results from the provision forincreased tolerance is that the degree of 'locking effect can be variedby varying the minimum diameter of the nut thread.

. For example, if the nut thread be gauged to the surface of the boltthread groove a very pronounced increase in locking effect will beobtained. By gauging the nut thread so that the crest of the thread ribwill extend slightly into the depression in the bolt thread grooveso'that there will be a slight interference at the shoulder l when thepartsiare brought into locking engagement there will be a still greater`locking effect.' l

It will be seen that very important practical advantages are obtained bya very simple improvement in the design of only one of the twocomplementary threads. This improvement, most desirably, entails only avminimum deviation from the design of standard, commercial Dardeletthreads. Only the root design ofthe external thread is' changed. Thedesign of the internal thread may remain unchanged from the standard,

as may also the general shape and proportions of both threads.

What I claim is:

1. A self-locking screw coupling including two members withcomplementary external and internal threads formed to affordclearancebetween their ribs for substantial crosswise relative axialshift when interfltted and with steep rib side faces to limit saidshift, all as in the known Dardelet type of screw coupling, a'helicoidal tolerance-augmenting channel formed in the bottom. of thethread groove of one of said threads and extending along one side faceof its thread rib, said thread groove having a thread-locking bottomsurface extending in a straight line widthwise from one margin of saidchannel to- 'y ward the opposite side face of the threadr rib across thethread groove and with a continuous crosswise slope with reference tothe thread axis within the angle of repose, and a single helicoidalsloping locking surface on the other thread and comprising the entirerib crest surface thereof and of less width than said channel and disoutof contact posable in radial register with the channel and limit of saidaxial shift, said sloping rib crest surfacebeing preformed to extend ina straight line crosswise of the rib and incline at the same angle withreference to the thread 'axis as said sloping thread-locking groovebottom surface of the other thread and in thev same direction to presentit for gradual, progressive, compressive wedging on said sloping surfaceand gradual increase of area of contact of said surfaces by relativeshiftl with the `bvottom thereof at one i of said crest surface axiallyout of radial register Y withxsaid channel to the opposite limit of saidaxial shift, the depths of the two threads being so proportioned withrelation to each other that saidcomprcssion will be within the elasticlimit of the metal in contact and be substantially uniform throughoutthe area in contact.

2. A self-locking screw coupling according to A V claim 1, characterizedin that the said tolerancechannel and associated 'sloping` augmentingthread-locking groove bottom surface are borneI byv the external one ofthe` complementary threads, the said locking rib crest surface forcoaction with said groove bottom surface is borne by the internalthread, and the tolerance-augmenting channel forms an abrupt depressionalong the low side of the sloping groove bottom locking surface.

3. A self-locking screw coupling according to claim 1, characterized inthatthe side wall of the said tolerance-augmenting channel which isadjacent to the said sloping groove bottom surface meets the adjacentedge of said surface at a steep angle and theI rib face of the otherthread whichv is nearest to said groove side wall in the unlockedrelation of the threads also meets the adjacent edge of the said crestlocking surface at a steep angle.

EDWARD J. MCMANUS.

